1. Field of the Invention
The present invention relates to an improvement in a low-section pneumatic radial tire. More specifically it relates to an improvement in high-speed durability of a low-section pneumatic radial tire.
2. Description of the Prior Art
In general, low-section (wide-tread) pneumatic radial tires with an aspect ratio (a ratio of section height to maximum width of a tire) of 0.3 to 0.6 are called ultrahigh performance tires, and suitable for use in high-speed vehicle travel.
FIG. 1 is a cross-sectional view for assistance in explaining an example of prior-art low-section pneumatic radial tires. In the drawing, the tire is formed with a cylindrical tread T extending between two tire sidewalls S which extend radially inward. The tire is roughly composed of a radial carcass 1 for reinforcing the tire from one sidewall S, through the tread T, to the other sidewall S, and a belt structure B having a width substantially the same as that of the tread T and disposed between the tread T and the radial carcass 1.
On the tread T, there are formed at least six circumferentially extending grooves made up of a pair of first innermost circumferential grooves 4A formed near the tire equatorial plane 0, a pair of second intermediate circumferential grooves 4B formed between the first inner most grooves 4A and both tread ends 6, and a pair of third outermost circumferential grooves 4C formed near the tread ends 6. These six circumferential grooves 4A, 4B and 4C divide the tread T into at least 7 lands including a first central land portion 5A, a pair of second intermediate land portions 5B, a pair of third intermediate land portions 5C, and a pair of fourth outermost land portions 5D.
The belt structure B is composed of at least two layers 2 formed by arranging inextensible metallic cords at a small inclination angle with respect to the tire equatorial plane 0 so that the cords of two different layers are laid one upon another in an intersectional relationship to each other. Three auxiliary layers are formed by arranging heat-shrinkable fiber cords on the outer circumferential surface of the belt layers 2 substantially in parallel to the tire equatorial plane 0.
The above-mentioned auxiliary layers 3 of the belt structure B is composed of a first auxiliary layer 3A extending over the entire width of the tire read T, a pair of second auxiliary layers 3B arranged at both side ends of the tire tread T in such away that the inner end of the layer 3B extends beyond under the outermost groove 4C, and a pair of third auxiliary layers 3C arranged at both side ends of the tire read T in such a way that the inner end of the layer 3C extends to a position slightly outward away from under the outermost groove 4C.
These three auxiliary layers 3 serve to prevent the belt layers 2 from expanding radially outward due to centrifugal force generated when the tire is rotating at high rotating speed, in order to improve the tire performance (durability) at high speed.
In the prior-art low-section pneumatic radial tire as described above, however, the inventors have found that there still exists a serious problem in that the belt layers are easily separated at both the tread side ends and/or the sidewalls are easily expanded into damage, in particular after the tire has been used to some extent and therefore worn off somewhat. In other words, the high-speed tire durability of the prior-art low-section pneumatic radial tire is still not satisfactory.
In summary, it has been known in the prior-art radial tire that the presence of the auxiliary layers 3A, 3B and 3C serves to prevent the tire tread from being expanded due to centrifugal force generated when the tire is rotated at high speed. However, it has not yet been noticed that the prior-art, radial tire involves a problem that the tire is extremely damaged after having been used to some extent.